The commercial utility of nanoparticles continues to grow at a rapid pace. Two examples of large and expanding markets for nanocrystalline and nanoparticulate titanium dioxide (also commonly called “titania”) are (1) optically transparent UV-absorbing coatings, i.e., coatings that absorb UV radiation but transmit visible light and (2) photocatalysis for air and water purification and in antibacterial applications. Major considerations for the titania/metal-oxide growth markets are to produce unique chemical and physical properties (such as highly reactive photochemistry for catalysis, and high optical transparency with low levels of photoreactivity for coatings) in combination with competitive manufacturing processes. The generation of new polymer/nanoparticle composites (“nanocomposite”), is an area with high potential that is currently the subject of intense investigation.
Large-scale commercial nanocrystalline TiO2 can be made from titanyl sulfate via acid-base chemistry, followed by calcination which provides highly agglomerated particles. A nanoscale material can be produced via the flame hydrolysis of TiCl4; however, this material is also highly agglomerated (typical agglomerate sizes are well over 300 nm). For numerous end-uses more finely divided particles are highly desirable. Since a significant amount of energy is required to reduce the size of the agglomerated particles, there has been a long felt need for a process which can synthesize finely divided titanium dioxide particles without the need for energy-intensive post-synthesis mechanical or chemical size reducing processes.
Titanium oxide particulates can be formed by hydrolyzing titanium alkoxides with water or a base. Titanium alkoxides are known to react rapidly with water to form a precipitate of agglomerated particles that must be milled to break-up the agglomerates to form small particles. JP [1989]-133939 describes a method for making titanium oxide particulates by dissolving titanium alkoxide in alcohol with a water content of 3 g/liter or less, and mixing the titanium-containing solution with an alcohol containing water and ammonia with the resultant mixture having specific mole-ratio ranges of NH3/Ti and H2O/Ti. EP 0275688 and U.S. Pat. No. 4,861,572 describe a first titanium alkoxide hydrolyzation step followed by a condensation reaction to form the metal oxide having particle sizes over 1 micron in size. JP 2001-246247 describes a subsequent acid or base treatment after hydrolysis of titanium alkoxide to produce relatively large particles of TiO2.
For optically transparent cosmetic or coating applications, it is important that particles not inhibit the transmission of visible light, which requires most of the particles to be smaller than about 100 nm.
It has been found that without adequate dispersion and colloidal stabilization, nanoparticles tend to form aggregates that are difficult to re-disperse and that affect optical properties in the final application. For some applications, it can be preferable for the nanoparticles to remain in suspension, preferably a suspension resulting from the synthesis, in order to preserve their size and hence their properties.
The process of this disclosure can provide an optically transparent suspension of titanium (IV) oxide nanoparticles.